Generally described, pulse oximetry is used to continuously monitor the arterial blood saturation of a patient in a variety of monitoring environments, such as hospitals, clinics, or for home use. Conventional pulse oximeters use two wavelengths of light generated from light emitting diodes (LEDs) that are transmitted into a pulsatile tissue bed corresponding to a tested subject. Generally described, for estimating oxygen saturation in the bloodstream, at least one of the LED's primary wavelength must be chosen at some point in the electromagnetic spectrum such that the absorption of oxyhemoglobin (HbO2) differs from the absorption of reduced hemoglobin (Hb). The second LED's primary wavelength is then chosen at some point within the electromagnetic spectrum such that the absorption of Hb and HbO2 are different from the absorption of the first LED. One skilled in the relevant art will appreciate that commercial pulse oximeters typically utilize one LED whose wavelength in the near red part of the electromagnetic spectrum near 660 nanometers. Additionally, the typical commercial pulse oximeter has a second LED whose wavelength is in the near infrared part of the electromagnetic spectrum.
The emitted light signals are collected by a photodiode, which processes the light signals into photocurrents. The photocurrents can be processed to measure a modulation ratio of the red, or near red signal, to the infrared, or near infrared signal. The modulation ratio then corresponds to arterial oxygen saturation (SaO2).
To properly calculate SaO2 levels, the wavelength of the LED's must be precisely known. One approach to transmit LED wavelength information utilizes a resistor with the pulse oximeter probe that has a value indicative of the wavelength of the LED. The resistor value of the LED is then used to code the transmission LEDs. The oximeter can read the resistor value and utilize the value of the resistor to calculate the SaO2 value. An example of a system utilizing this approach can be found in U.S. Pat. No. 4,621,643, assigned to Nellcor, Inc. Although this approach allows for increased tuning, the approach can become deficient in that it typically requires separate electrical connections to read the resistor value. Because each separate electrical connection increases the overall cost of the unit, this approach is not cost effective.
Another approach to improve the transmission of LED wavelength information also utilizes a resistor having a value indicative of the wavelength of the LED. However, this approach places the resistor in parallel to the LED so that it does not significantly add to the cost of the pulse oximeter. By providing a current that does not enable the LED, an oximeter can read the information from the parallel resistor and process the wavelength information. An example of a pulse oximeter utilizing this approach can be found in U.S. Pat. No. 6,011,986. However, this approach is typically limited in that the dynamic range of reading voltages can never exceed the turn on voltage of the LED. For example, this approach can be limited to the utilization of resistors because other identification devices cannot typically operate at a level below the operating voltage of the LED.
Thus, there is a need for a pulse oximeter configuration that can transmit information corresponding to the operation of the pulse oximeter without requiring additional wiring components and/or that allows for a full range of reading environments.